The scoop: Sleeping on your back makes it easy for your head, neck, and spine to maintain a neutral position. You’re not forcing any extra curves into your back, says Steven Diamant, a chiropractor in New York City. It’s also ideal for fighting acid reflux, Dr. Olson says: "If the head is elevated, your stomach will be below your esophagus so acid or food can’t come back up."

Back-sleeping also helps prevent wrinkles, because nothing is pushing against your face, notes Dee Anna Glaser, MD, a professor of dermatology at Saint Louis University. And the weight of your breasts is fully supported, reducing sagginess.

The scoop: Side-sleeping is great for overall health—it reduces snoring and keeps your spine elongated. If you suffer from acid reflux, this is the next best thing to sleeping on your back. The downside: "Sleeping on your side can cause you to get wrinkles," Dr. Glaser says. Blame all that smushing of one side of your face into the pillow.

If you’re pregnant, sleep on your left side. It’s ideal for blood flow.

Perfect pillow: A thick one. "You need to fill the space above your shoulder so your head and neck are supported in a neutral position," says Ken Shannon, a physical therapist at Brigham and Women’s Hospital in Boston.

The scoop: When you snooze with your knees pulled up high and chin tucked into your chest, you may feel it in the morning, especially if you have an arthritic back or joints, Dr. Olson says.

"This curved position also restricts diaphragmatic breathing," adds Dody Chang, a licensed acupuncturist with the Center for Integrative Medicine at Greenwich Hospital in Connecticut. And if you make this your nightly pose, you may bring on premature facial wrinkles and breast sag.

The scoop: "Stomach-sleeping makes it difficult to maintain a neutral position with your spine," Shannon explains. It puts pressure on joints and muscles, which can irritate nerves and lead to pain, numbness, and tingling. "Think about the soreness you’d feel if you kept your neck turned to one side for 15 minutes during the day," Dr. Diamant explains.

In this position you have your head to one side for hours at a time. You won’t necessarily feel it the next day, but you may soon start to ache.

Do you snore? "Stomach-sleeping may even be good for you," Dr. Olson says. Facedown keeps your upper airways more open. So if you snore and aren’t suffering from neck or back pain, it’s fine to try sleeping on your belly.

Think again. While you generally spend the most time in the position you fall asleep in, even those who barely have to make their beds in the morning move two to four times an hour, which may add up to 20 or more tosses and turns a night, says Eric Olson, MD, co-director of the Mayo Clinic Center for Sleep Medicine in Rochester, Minnesota. "That’s completely normal, and you’ll still go into deep REM sleep, the restorative kind," he says.